customModelAdminSearchContext.php

<?php

class YourModelAdmin extends ModelAdmin{

/**
     * @return SearchContext
     */
    public function getSearchContext()
    {
        $context = parent::getSearchContext();
        if ($this->modelClass == 'YourClass') {
          $newField = DropdownField::create('q[FieldName]');
            $context->getFields()->replaceField('YourFieldToReplace', $newField);
        }
        return $context;
    }
    
}